1. Field of the Invention
This invention relates to multiple transmit zone splicing in an ultrasound imaging system.
2. Description of Related Art
Ultrasound imaging systems generally operate by transmitting ultrasound signals from an ultrasound transducer or a plurality of transducer elements into a human body at a skin surface or within a body cavity, and receiving ultrasound signals reflected by objects or structures, such as organ tissue or other acoustic interfaces in a scan region (such as a scan plane), back to the ultrasound transducer.
The transmitted ultrasound signals form an acoustic scan line, generally focused at a focal point on the scan line. The reflected ultrasound signals which are received are processed to generate successive sample points representing reflected acoustic energy along the scan line. A region of samples along the received scan line, within which the ultrasound energy was focused when transmitted is called a "transmit zone".
One problem which has arisen in the art is where there are multiple transmit zones in a scan line, i.e., where a single scan line comprises two or more zones in which ultrasound energy is independently focused. Multiple transmit zones may be necessary or convenient, for example, to improve resolution through a depth range to be imaged. When combining multiple transmit zones, boundary delineation between zones due to differences in the focus and apparent gain within the zones, may be apparent to an operator and produce an undesirable artifact in an ultrasound image.
One method of the prior art has been to simply abut adjacent transmit zones in range. While this method of the prior art achieves the goal of constructing an entire scan line image using multiple transmit zones, it suffers from the drawback that zone boundaries are readily apparent in a large number of imaging situations, due to focus and gain variations between the transmit zones. The appearance of the zone boundary is an undesirable artifact in the displayed image.
Another method of the prior art has been to stagger the transmit zone boundary for alternate scan lines, and to average pairs of adjacent scan lines. While this method of the prior art achieves the goal of constructing an entire scan line image using multiple transmit zones, it suffers from the drawback that averaging pairs of adjacent scan lines reduces cross-range resolution (e.g., azimuthal resolution). Moreover, this method of the prior art has the drawback that it is not amenable to architectures which employ cross-range interpolation (e.g., azimuthal interpolation) to generate one or more interpolated scan lines between real scan lines, prior to the step of averaging adjacent scan lines.
U.S. Pat. No. 5,113,706, "Ultrasound System with Dynamic Transmit Focus", issued May 19, 1992, in the name of Richard J. Pittaro, and assigned to Hewlett-Packard Company, shows an ultrasound system in which focal points and gain are stepped through a series of focal zones, so as to piece together a field larger than a single transmit zone. While the method shown by this patent may achieve large depth of field, it does not solve the problem presented by multiple transmit zones, and still suffers from the drawback that there may be a significant discontinuity at points where there is a significant gain change or focusing characteristic change.